There are a large number of solid-state circuit breaker and renewable energy applications that require semiconductor device configurations capable of bi-directional power flow. Some examples if such applications include:
1. Solid-state circuit breakers
Provide bidirectional fault isolation in a fraction of a microsecond, in power electronic circuits such as those used in industrial applications and hybrid vehicles. The mechanical contactors used presently are too slow and suffer severe degradation during repeated fault isolation.
This technology is actively sought by the Army for their hybrid military ground vehicles, by the Air-Force for their latest generation fighter-aircraft 270 DC power system, and by the navy for their high-voltage ship systems. Solid-state (semiconductor) circuit breakers will eventually be present in every hybrid vehicle and every industrial power circuit that needs fault protection.
2. Bidirectional power flow enables regeneration applications
Industrial applications can reap significant energy savings by returning otherwise wasted energy to the AC mains. Examples include rolling mills, conveyor belts, and elevators. In electric-gasoline hybrid vehicles, bidirectional power flow is needed for the battery to provide a cold start and to receive and store the regenerated energy from braking.
3. Photovoltaic bidirectional power transfer to the utility grid and batteries
Bidirectional flow of solar photovoltaic power to the utility AC grid and from the utility AC grid to charge the photovoltaic battery bank.
4. Wind power transfer to the utility grid and back
Bidirectional power flow between the wind turbine generator and the utility grid.
5. Fuel cell bidirectional power flow
In fuel cell hybrid vehicles the electric drive-train motor is supplied by an inverter connected to a fuel cell. In addition, traditional chemical-power batteries are employed to provide better cold start characteristics and the option to recover braking energy. Bidirectional power flow is needed to interface the chemical-power battery with the fuel cells.
Bidirectional power flow requires:
1. Symmetric current flow in forward and reverse directions
2. Blocking of a specified voltage in forward and reverse directions
Currently, bidirectional, semiconductor power flow applications make use of silicon devices. These include MOSFETs, IGBTs, BJTs, and JFETs. Reliable operation of silicon devices is temperature limited to below 120° C. With respect to wide-bandgap semiconductors devices, silicon devices have a larger physical size (footprint), operate at lower frequencies, and are more resistive. Accordingly, silicon devices experience higher switching and conduction losses, which result in lower system efficiency. An additional drawback of silicon devices is their lower short-circuit hold-off time, i.e., they experience catastrophic failure much faster than wide bandgap devices.
Devices made out of SiC, GaN and other wide-bandgap semiconductors can operate reliably at +300° C., have lower conduction and switching losses, have a smaller physical size, and a longer short-circuit hold-off time. In addition, due to their lower switching losses, wide-bandgap semiconductor devices operate at high frequencies unattainable by silicon which greatly simplifies circuit design and eliminates a number of filters/elements.
Several SiC power devices are candidates for bidirectional power conditioning applications:                SiC BJTs: suffer from forward voltage degradation (reliability issue), and expensive specialized epitaxy wafers must be used to alleviate this problem. BJTs have a relatively low current-gain that complicates gate-drive design and increases gate-drive size and losses. Also, the BJT current-gain deteriorates with temperature and that limits operation to below 200° C. BJTs have a negative temperature coefficient. This raises thermal run-off concerns when paralleling multiple devices to meet the high-current requirements of modern power conditioning systems.        SiC MOSFETs: suffer from low mobility and reliability stemming from its native gate oxide. MOSFETs temperature operational range is limited to ˜200° C. due to gate oxide instability and threshold-voltage-shift with temperature. MOSFETs are also complicated devices to fabricate in SiC resulting in increased manufacturing costs.        Lateral-channel vertical JFETs: Have higher resistance than that of vertical-channel JFETs (VJFETs), which increases losses and limits bandwidth. Fabrication is more complicated than that of a VJFET.        Vertical-channel JFETs (VJFETs): no gate-oxide or forward-voltage-degradation reliability concerns, have been operated above 300° C., are voltage-controlled devices when operated in unipolar mode, and are relatively easy to fabricate. At this time, the VJFET is the most mature SiC power transistor.        
Given the compelling high-frequency switching, high temperature operation, low-resistance, fabrication-simplicity, long short-circuit hold-off time, and reliability advantages of SiC VJFETs, design techniques for optimal VJFET operation in bidirectional power flow circuits are needed.
For doping levels relevant to power conditioning applications, the wide band-gap of SiC power devices leads to gate-to-source and gate-to-drain pn junction built-in potentials (turn-on) of about 2.7 V, as shown in FIG. 1. FIG. 1 shows VJFET forward gate-to-source (filled triangles) and gate-to-drain (open circles) pn junction characteristics. An important requirement for efficient power-VJFET gate-drive operation is maintaining voltage-control capability by having the gate-to-source and gate-to-drain pn junctions operate below their built-in potential values, i.e., unipolar VJFET operation. This biasing condition also contributes to VJFET reliability.
It was shown in U.S. application Ser. No. 12/623,655, entitled SYSTEM AND METHOD FOR PROVIDING SYMMETRIC EFFICIENT BI-DIRECTIONAL POWER FLOW AND POWER CONDITIONING, which is hereby incorporated by reference, that two VJFETs connected in common source configuration can achieve efficient (unipolar, VJFET pn junctions do not turn on) symmetric bidirectional power flow operation, under the gate bias conditions below (where VGS is the common gate-to-source bias voltage, VD2S is the drain-to-source bias voltage for the second VJFET and VD1S is the drain-to-source bias voltage for the first VJFET):VGS≦2.5 V−|VD2S| and VGS≦2.5 V−|VD1S|  (1)
Typical on-state drain current characteristics vs. drain voltage of a high-voltage normally-on VJFET, at a gate-to-source bias range of 0 to 3 V in steps of 0.5 V, are shown in FIG. 2. Typical on-state drain current characteristics vs. drain voltage for the high-voltage VJFET of FIG. 2, at a gate-to-source bias range of 0 to −4.5 V in steps of 0.5 V, are shown in FIG. 3. At a gate-to-source bias of −4.5 V, the VJFET's channel is pinched-off and negligible current flows through the VJFET's drain. It is evident from FIGS. 2 and 3, that operating the VJFET at a unipolar gate bias of VGS=2.5 V allows for maximum drain current output without the deleterious effects of turning on the gate-to-source pn junction. In addition, FIGS. 2 and 3 show that reducing the VGS gate bias leads to an undesirable reduction in current output. From Eq. (1) above, it is clear that VJFETs connected in common source configuration for efficient (unipolar) symmetric bidirectional power flow will always operate below the gate bias of VGS=2.5 V that maximizes their current output. Also, in applications like solid-state circuit breakers where the VJFETs are in the on-state for the vast majority of time, it is desirable that the on-state corresponds to a passive VGS=0 V so that conduction is maintained with no active bias applied to the gate.
Modern power conditioning applications require high levels of current and numerous devices need to be paralleled. From the above, it is seen that VJFET design techniques that maximize drain current as the VJFET gate bias VGS drops below its maximum unipolar value of about 2.7 V are needed. Design techniques than enable VJFETs to achieve optimal operation in bi-directional circuits per the above are needed.